This invention relates to a drive wheel suspension system for a motorcycle of the kind in which the drive wheel is driven by a chain cooperating with sprockets provided on the engine shaft and on the wheel shaft. In such motorcycles, it has been common practice to employ a swing arm for mounting the wheel, the swing arm having a pivot usually located in the region between the engine sprocket and the periphery of the driven wheel. With this configuration, the motion of the swing arm, which accompanies deflection of the wheel, results in change in the length of the path to be followed by the drive chain interconnecting the engine and wheel sprockets; and a chain slack take up device is commonly employed in order to maintain proper driving engagement of the chain with the sprockets. In addition, with this prior arrangement, certain undesirable forces are present in the motorcycle, especially during acceleration and braking. One such undesirable force is commonly referred to as "squat", i.e., a force tending to cause the rider's seat to deflect downwardly during acceleration.
It has also been known (see for example U.S. Pat. No. 4,058,181) to provide a parallelogram type of swing arm suspension arrangement for mounting the drive wheel, this arrangement, in effect, comprising multiple swing arms located above and below the axes of the driving sprockets, the multiple swing arms being interconnected and arranged to provide for pivotal motion of the drive wheel about the axis of the engine sprocket, thereby eliminating the change in the length of the chain path. Although this makes possible elimination of the chain slack take up device, the prior art arrangement has a number of disadvantages, notably the fact that the multiple swing arm elements used for the parallelogram arrangement of said prior patent extensively increase the mass of the parts which must move with the wheel. Moreover, with the multiple swing arms located above and below the axes of the sprockets, certain undesirable forces during acceleration and braking are still present in the overall structure of the motorcycle, and, in fact, are even exaggerated.
According to the present invention, provision is made for a parallelogram linkage adapted to be associated with the inner or pivoted end of the swing arm preferably in a region between the periphery of the wheel and the axis of the engine sprocket, but arranged to provide an effective swing arm axis which is coincident with the axis of the engine sprocket. By locating the linkage parts in the region between the periphery of the wheel and the axis of the engine sprocket, the weight of the parts involved is concentrated relatively close to the effective axis of swing motion. This greatly reduces the overall weight of the parts and also reduces the inertia of the parts moving with the wheel and, consequently, improves the responsiveness of the wheel under various operating conditions including, for example, travel over rough terrain such as is commonly encountered in motocross racing. Still further, the arrangement of the invention provides for use of a swing arm structure lying substantially in a plane containing the axes of the driving and driven sprockets, and this reduces certain of the undesirable forces which tend to occur especially during acceleration.
The invention also contemplates association of the deflection resisting mechanism, such as a typical shock absorbing or shock strut device, with the parallelogram linkage parts located close to the effective swing motion axis, these parts being arranged to provide a reaction point on the frame of the motorcycle which is relatively close to the overall center of gravity of the motorcycle, thereby minimizing tendency for the action of the shock strut to cause the attitude of the motorcycle as a whole to change in consequence of substantial wheel deflection.
The invention also provides several alternative parallelogram suspension arrangements having the advantages hereinabove mentioned, but still further providing additional advantages. In one such alternative embodiment, multiple levers and links are employed in order to increase the overall strength of the linkage. In another aspect of the alternative embodiments, provision is made for alternative location of certain of the parts of the linkage mechanism, thereby readily adapting the parallelogram arrangement of the invention to motorcycles of different construction, as will be explained hereinafter.
Still further, with linkage and swing arm arrangements of the kind herein disclosed, various of the advantages above referred to may be attained, even when certain of the linkage elements do not precisely define a parallelogram linkage. In view of this, greater flexibility is provided with respect to the location of the linkage pivots on the motorcycle frame.